The “unmixing of peoples” in the Ottoman-Russian borderlands was a century-long and traumatic process, both violent and voluntary, state-led and local. Most historical literature has naturally focused on the genocidal deportations in these borderlands during the First and Second World Wars. The logic of ethnic unmixing, however, was inherent to the idea of the modern state and extended into the peacetime laws and practices of the early Turkish Republic and Soviet Union. This paper examines the formation and implementation of a new, putatively “rational” population policy in the Turkish-Soviet border region in the mid-twentieth century. The paper covers three interrelated topics: resettlement policies, new controls over trans-border movement, and the use of internal exile as a means of social engineering. In addition to being inspired by “legal-rational” ideologies of domestic ethnic homogenization and population control, such ideologies, this paper demonstrates, transcended state boundaries, and population policies were actually coordinated by the Turkish Republic and the Soviet Union. Archival materials from Turkey and the Caucasus, alongside published memoirs, the contemporary press, and family histories constitute this paper’s source material.